What would be a fair way to adjudicate the detonation of a grenade in an enclosed space, such as a stairwell?

I have used the rules for blowback in games such as Shadowrun before, but recently ran a scene in an A Time of War session where a soldier lobbed grenades after a fleeing group of PCs as they ran down several flights of stairs. Due to the circumstances of the scene, the detonation did not affect anyone, but what if it had?

If the grenade exploded in a narrow flight of stairs, what would happen to the force and shape of the blast? If the blast was defined as being 12 points of damage at the point of detonation, and decreasing 1 point per circular meter it expands outward from that point, what would be an appropriate modification for the blast if it occurred in a space 2m wide and 3m high, with flights of stairs spanning 4m of horizontal distance and dropping 4m within that span.

(I recognize that these figures may not actually represent a sane flight of stairs, please indicate more appropriate stair stats in your answer if you like. I present these here as these are the dimensions I used in the session. )

Also, it should be clarified that I am interested in all aspects of the explosion, including but not limited to increased damage, increased range where the blast can escape, secondary effects such as deafness, dust clouds, etc.

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    \$\begingroup\$ You say you have used the Shadowrun rules before... but apparently found them inadequate to model this specific scenario. Why? According to Shadowrun rules, the effect would be pretty much lethal (over 3 times normal grenade damage, without going into detailed calculations), as would be realistic. Apparently, that is not what you are looking for. Are you looking for a more "realistic" (probably even more lethal) model, or something "fair" (i.e., letting your PCs get away)? \$\endgroup\$
    – DevSolar
    Jul 6, 2012 at 11:04
  • \$\begingroup\$ I've got to agree with DevSolar... I ran a Shadowrun game long, long ago (in the days of 1st edition rules) and one fight involved a grenade blast in a stairwell. Shadowrun's "chunky salsa effect" rule lived up to its name and was quite satisfactorily brutal to the guards in that stairwell. \$\endgroup\$ Jul 6, 2012 at 11:15
  • \$\begingroup\$ It is not that I find them inadequate, it's that they are pretty much my only insight into this sort of mechanic and in the spirit of skill expansion, I am curious about options, things which should be considered, and the best way to resolve the simulation. I was not playing SR at the time, but mechwarrior, and so did not choose to apply the rules from SR if things required damage to be applied. In all cases, I look for the most realistic means of simulating effects in a game world as its underlying concepts allow. \$\endgroup\$ Jul 6, 2012 at 11:24
  • \$\begingroup\$ Ah the classic "30 foot Fireball spell in a 10 Foot room" crux. One method depends on if you're worried about those in the AoE being able to dodge, and another depends on if you're concerned about the damage itself. \$\endgroup\$
    – CatLord
    Jul 6, 2012 at 19:00
  • \$\begingroup\$ @CatLord I am actually concerned with what is the most appropriate way to simulate the damage given its containment. Is it worthwhile to calculate an increase in damage and/or range at the two points where the force is nit contained, or should it just be resolved as though there were no containment? \$\endgroup\$ Jul 15, 2012 at 23:31

2 Answers 2


Theory: This article Injuries From Explosions: Physics, Biophysics, Pathology, and Required Research Focus may have some useful information. The Journal of trauma will definitely have information about it, it's just a matter of looking – possibly from academia otherwise you may need to pay. Conventional Warfare Ballistic, Blast and Burn Injuries, Chapter 7 looks like a good starting point on the physics of blasts.

But what does this all tell you about your stairwell?

Practice: Now that you know the real life effects of explosive blasts, you can model it in your game. You can split it into a few parts:

  1. Realistic: Chances are that everyone is concussed, disoriented, and may have shrapnel in them. Depending on stairs materials, there may be structural damage. This structural damage may form more shrapnel and thus cause more damage – wooden stairs could splinter horribly. Note that this is the only time when the academic knowledge is needed and useful.
  2. Hollywood: Everyone is covered in dust, ears may ring a little.
  3. Hong Kong: PCs redirect the blast with their ki to blow the doors at the bottom of the stairs and escape.
  4. Narrative: What effect of said explosion would make the story more interesting/dramatic?
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    \$\begingroup\$ Great answer, and thanks for collecting those links. Research begins anew~ \$\endgroup\$ Jul 6, 2012 at 11:36
  • \$\begingroup\$ The Journal of trauma is a superb source of material for working out what damage does to the human body. Some of the police and army lesson learned are not bad as well but are harder to find. \$\endgroup\$ Jul 6, 2012 at 11:47
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    \$\begingroup\$ Wound shock is something to take into effect as well with just about any form of physical harm in real life. Cyberpunk 2020 had a shock roll every time you took damage in the Friday Night Firefight system. \$\endgroup\$ Jul 6, 2012 at 12:43
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    \$\begingroup\$ Excellent, thorough answer. I'd add that if you're going with the realistic option, the effects of concussion and disorientation are quite intense. Even having just your hearing overloaded is profoundly jarring. It's difficult to think straight, difficult to react, and the effect can take hours to taper off. \$\endgroup\$ Jul 6, 2012 at 17:54
  • \$\begingroup\$ @Joshua Aslan Smith A Time of War, the mechwarrior rpg which sparked this question, takes shock and fatigue into account when characters are wounded. I agree it is an important element. \$\endgroup\$ Jul 15, 2012 at 23:26

As a rough rule of thumb, I'd count the blast reflecting off a sufficiently solid wall as being (roughly) a metre of distance, then simply overlap the distances.

However, shrapnel would behave differently. So it also depends on the grenade. Some are primarily over-pressure devices, some are primarily shrapnel-based and most are hybrid (that is, some of the damage is intended to be from over-pressure and the bulk from shrapnel, it depends crucially on the design of the grenade). All in all, this simplified model should be a decent trade-off.

In real life, it's more complicated than that, but the process I outlined should be at least a step closer to realistic (and leave 30-40 steps to take, but you don't want to be running full-on physics simulations every time you need to resolve a particular conundrum).

  • \$\begingroup\$ What do you suggest would happen at the open ends of the flight of stairs where things widen out into the landings at each floor? \$\endgroup\$ Jul 6, 2012 at 11:37
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    \$\begingroup\$ @Runeslinger - I would expect getting a blast front opening up in a roughly 60 degree cone, with shrapnel being in a cone with its point at the grenade's blast position, delimited by straight lines cutting the corners. Unfortunately, all the supporting material I can think of is primarily in Swedish (and may also be restricted). \$\endgroup\$
    – Vatine
    Jul 6, 2012 at 12:09

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